The present invention relates to a light beam scanning method and apparatus.
In more detail, the present invention relates to a light beam scanning method and apparatus that realize high-precision light scanning by compensating for an error in a condensing spot position due to lateral displacement of a light beam and longitudinal displacement (defocus) of the light beam caused by various factors and degradation (blurring) in a condensing spot shape due to aberration in an optical system. That is, the present invention relates to a light beam scanning method and apparatus that uses a light scanning technique with which there is performed high-quality image recording or high-precision image reading.
In particular, the present invention relates to a cylindrical internal surface scanning type light beam scanning method and apparatus that use a light scanning technique with which during cylindrical internal surface scanning type light beam scanning, there is compensated for at least one of lateral displacement of a light beam, longitudinal displacement of the light beam, and aberration in an optical system caused by various factors and there is performed high-quality image recording or high-precision image reading.
Also, the present invention relates to a cylindrical external surface scanning type light beam scanning method and apparatus that use a light beam spot compensating light scanning technique that realizes high-precision light scanning by compensating for at least one of lateral displacement of a light beam, longitudinal displacement of the light beam, and aberration in an optical system caused by various factors or the changing of an environmental temperature, with the cylindrical external surface scanning type light beam scanning method and apparatus being suitably applicable to any optical recording head such as a CTP (Computer to Plate), an image setter, or a DDCP (Direct Digital Color Proofer). Alternatively, the present invention relates to a light beam scanning method and apparatus that use a light beam spot temperature compensating light scanning technique that is suitably applicable to any of the light recording heads described above.
Also, the present invention particularly relates to a cylindrical internal surface scanning type light beam scanning apparatus that functions as a cylindrical internal surface scanning type image recording apparatus that records an image by performing main scanning on a recording medium, such as a photosensitive material, held on a cylindrical internal surface using a plurality of light beams modulated in accordance with image information.
There have been developed light beam scanning apparatuses such as a light beam reading apparatus that performs the reading of an image using a light beam and a laser printer that records an image by guiding a light beam from a laser or the like, onto a recording medium using a scanning optical system and by exposing the recording medium through the scanning of the light beam. These light beam scanning apparatuses include, in the category thereof, a flat bed type that scans a laser beam on a plane-shaped recording medium, a cylindrical external surface scanning type (external surface cylinder type (outer drum type)) that scans a laser beam on a recording medium placed on an outer peripheral surface of a rotating drum, and a cylindrical internal surface scanning type (internal surface cylinder type (inner drum type)) that scans a laser beam on a recording medium placed on the inner peripheral surface of a drum.
Among these, the inner drum type light beam scanning apparatus does not suffer from the peeling of a recording medium during recording and excels in high-speed scanning ability, cost efficiency, and the like. Also, the quality of a recording light beam is high, so that the inner drum type light beam scanning apparatus is widely used.
A conventional inner drum type light beam scanning apparatus is shown in FIG. 20. As shown in FIG. 20, the inner drum type light beam scanning apparatus 500 is basically constructed of a drum 504 on whose cylindrical internal peripheral surface there is placed a recording medium (recording sheet) 502 on which an image or the like is to be recorded, a light source (laser beam generator) 506 that generates a laser beam L, and a laser beam scanning portion 508 that is disposed concentrically with the center axis of the drum 504 and scans the laser beam L on the recording medium 502.
The laser beam scanning portion 508 includes a condensing lens 510 that is set on the same optical axis as the laser beam L, a light deflector (spinner) 512 that has a reflection surface set at an inclination angle of substantially 45° with respect to the incident direction of the laser beam L, and a motor 514 that rotationally drives the spinner 512 about the optical axis of the laser beam L.
The laser beam L outputted from the laser beam generator 506 is then condensed by the condensing lens 510, is reflected by the reflection surface of the spinner 512 that is rotated by the motor 514 at high speed, and is guided onto the recording medium 502. During this operation, the laser beam L is scanned (main-scanned) by the rotating spinner 512 while rotating on the recording medium 502 at high speed. Also, the laser beam scanning portion 508 is moved for auxiliary scanning in a direction shown by the arrow X in the drawing by an unillustrated traverse that moves for auxiliary scanning. In this manner, an image or the like is recorded on the recording medium 502.
In the inner drum type light beam scanning apparatus, however, there is a problem in that because the spinner that is a deflector is rotated, its reflection surface is distorted by a centrifugal force, the wave front of the laser beam that scans the recording medium is distorted, and the quality of a recorded image is lowered.
In view of this problem, U.S. Pat. No. 5,907,153 discloses a technique of compensating for distortion in a reflected beam caused by distortion in the reflection surface due to the rotation of this spinner using an optical path length compensating device of a transmission type or a reflection type.
That is, as shown in FIG. 21, in the light beam scanning type image recording apparatus, a laser beam generated from a laser diode 601 connected to a modulator 602 and passed through a light level controller 603 is reflected by a compensating device 604, passes trough a beam expander 605, a resolution selector 606, a focusing lens 607, and the like, is reflected by a reflection surface 610 of a rotating spinner 611, and is scanned on a recording medium 608 placed on the internal surface of a cylinder 609, thereby recording an image or the like. During this operation, there is made an attempt to compensate for aberration or distortion caused by the rotation of the spinner 611 with this compensating device 604.
As shown in FIG. 22, the compensating device 604 includes an electrode 615 and a piezo device 614 that are disposed under a plane-shaped mirror 613. As shown in FIG. 23, the compensating device 604 further includes electrodes 616A to 616L that are divided into 612 portions ant are disposed in a circumferential manner under the electrode 615 and the piezo device 614. As shown in FIG. 21, the electrodes 615 and 616A to 616L are controlled by a memory 619, a microprocessor 618, and a voltage source 620.
Also, by deforming the piezo device 614 through the control of the electrodes 616A to 616L in synchronism with the rotation of the spinner 611, the distortion in the laser beam caused by the spinner 611 is compensated for through the deforming of the mirror 613.
However, as causes of the degradation in the quality of a recorded image or the degradation in a figure accuracy of image reading, aside from the aforementioned distortion in the laser beam, there are cited the increase of a spot diameter, the degradation in a condensing spot shape such as the collapse of a spot diameter, and lateral displacement of a light beam where the position of a dot for recording an image is shifted from a target position (that is, an error in the condensing spot position). The cause of the degradation in the condensing spot shape and the error in the condensing spot position is not limited to the aforementioned distortion in the rotating spinner. That is, in addition to causes resulting from the spinner, there are cited, for instance, causes resulting from longitudinal displacement (defocus) of a light beam, aberration in an optical system, and the like. The causes due to the spinner are dynamic, while the causes due to the aberration in the optical system are caused by a lens itself or the combination of lenses and are static. Also, in addition to the above, there are cited causes such as an error from the roundness of the drum (eccentricity of the drum or a cylindrical error), a deviation of a traverse that is moved for auxiliary scanning from a straight line property, a mismatch between the moving direction of the traverse and the center line of the drum (parallel shifting or crossing), or a variation in the thickness of a recording medium.
Here, in short, the degradation in the condensing spot shape refers to longitudinal displacement (defocus) of a laser beam in an optical axis direction (z direction), the increase of a spot diameter due to aberration in an optical system, and the collapse of a spot shape. Also, the error in the condensing spot position refers to displacement of the recording position of a laser beam on the recording medium (in the x and y directions).
Up to now, in order to circumvent the degradation in the condensing spot shape caused by the aforementioned various factors (hereinafter represented by the longitudinal displacement of the light beam and the aberration in the optical system) and the error in the condensing spot position (hereinafter represented by the lateral displacement of the light beam) and in order to obtain high-quality recorded image, there is no choice other than relying on high-precision processing or high-precision adjustment, which eventually leads to a problem in that the cost of the apparatus is increased.
Incidentally, as described above, as one light beam scanning apparatus, there is a cylindrical external surface scanning type light beam scanning apparatus.
As described above, this apparatus is also referred to as the outer drum type. In this apparatus, a recording beam is imaged on a recording medium wound around the external peripheral surface of a drum (recording drum) that rotates at constant speed, and at the same time, a scanning optical system is moved for the auxiliary scanning in the rotation axis direction of the drum, thereby scanning the recording medium with a light beam.
Up to now, there has been known that in the outer drum type light beam scanning apparatus, the lateral displacement of a recording light beam, blurring due to aberration in the optical system, and longitudinal displacement of the recording light beam are caused by various factors to be described below and image quality is degraded.
For instance, due to the eccentricity or distortion in a cylindrical direction caused during the manufacturing of a drum, a variation in the thickness of a recording medium, such as a plate or a film, wound around the drum external peripheral surface, and the like, there occurs lateral displacement of a light beam or longitudinal displacement of the light beam on a recording surface while the drum is making one rotation.
Such lateral displacement of the light beam on the recording surface is also caused by the distortion or bending in a rail of an auxiliary scanning mechanism that has the scanning optical system perform auxiliary scanning, the bending in an exposure surface plate, an error in the auxiliary scanning sending speed due to an error in a lead pitch of an auxiliary scanning ball screw of the auxiliary scanning mechanism, or the like.
Further, shifting of a figure is caused by displacement of a recording beam caused during the spiral exposure using a multi-beam. As described above, due to various factors, there occur lateral displacement of a recording beam and degradation in image quality such as figure accuracy. Up to now, these problems have been coped with only by processing each construction element, such as a drum, with high precision. This means that there has been taken no effective measure.
Also, in the case of the CTP or the like, a plate made of aluminum is wound around the external surface of a drum. During the rotation of the drum, this plate is pulled outwardly by a centrifugal force and is placed in a state where the plate is floated from the drum external surface, which causes longitudinal displacement. Alternatively, in the case where minute dust is sandwiched between a recording medium, such as a plate or a film, wound around the drum external surface and the drum external surface, the recording medium is floated from the drum external surface at a corresponding position and there occurs longitudinal displacement in a like manner. Further, if the rail of the auxiliary scanning mechanism (traverse) for moving the scanning optical system for auxiliary scanning in the rotation direction of a drum is distorted or bent, there occurs longitudinal displacement in accordance with the auxiliary scanning.
Up to now, such longitudinal displacement is compensated for by moving the position of a lens system for imaging light emitted from a light source.
As shown in FIG. 24, for instance, a light beam emitted from a light source 700 is guided onto a recording medium 708 wound around the external surface of a recording drum 706 through a first lens group 702 and a second lens group 704. In an exposing apparatus that performs scan exposure, the second lens group 704 is moved as indicated by the broken line in the drawing. Thus, the aforementioned longitudinal displacement is compensated for, thereby correcting the point of a recorded image.
Alternatively, as shown in FIG. 25, by moving the whole of the optical system including the light source 700, the first lens group 702, and the second lens group 704, the longitudinal displacement is compensated for.
However, there is a problem in that if there is made an attempt to compensate for the lateral displacement of a light beam by relying on the high-precision processing of a drum and the like, the product price is raised.
Also, as described above, with the method with which the longitudinal displacement of a light beam is compensated for by moving a lens group and the like, there are moved the heavy-weighted lens group and the like, so that it is impossible to expect high-speed response, and for instance, it has been extremely difficult to compensate for longitudinal displacement resulting from the aforementioned causes such as dust. Also, the lens group is mechanically moved, so that there is a danger that vibrations caused by the mechanical moving exert an adverse effect on recording accuracy.
Consequently, in order to eliminate the lateral displacement or longitudinal displacement of a light beam, it has conventionally been required to process each construction element like a drum with high precision, to add a device that removes dust and the like, and to take other measures, which causes a problem in that the price of a product is raised.
Also, there has conventionally been known that if an environmental temperature is changed during the light scanning in these light beam scanning apparatuses, lateral displacement of a recording light beam (displacement of a recording position on the recording medium surface) and longitudinal displacement of the recording light beam occur due to various factors to be described below, which leads to the degradation in image quality.
In the case of the outer drum type, for instance, the lateral displacement and longitudinal displacement of a recording beam are caused by the degradation in figure accuracy, such as the magnification accuracy or registration accuracy, caused by the expansion or the shrinking of a drum in its diameter direction due to the changing of the environmental temperature, the elongation of a surface plate supporting the drum or an auxiliary scan exposure head in the optical axis direction, or the like.
In the case of the inner drum type, the lateral displacement of the recording beam is caused by the shrinking in the drum diameter direction due to the changing of the environmental temperature, tilt of a lens, displacement of a spinner mirror, or the like. Also, longitudinal displacement of a recording beam is caused by the changing of a focal distance of a lens glass material due to the changing of the environmental temperature, the shrinking in the drum diameter direction, the changing of the position of a spinner in the optical axis direction, or the like.
Also, in the case of a flat bed type, lateral displacement of a recording beam is caused by displacement of a light deflector due to the changing of the environmental temperature, the elongation of a flat bed, the tilt shifting of a long mirror, or the like. Also, longitudinal displacement of a recording beam is caused by the fluctuations of a distance between lenses on an optical surface plate due to the changing of the environmental temperature, the changing of a focal distance of a lens glass material, the displacement of a long mirror in the optical axis direction, or the like.
Such lateral displacement of a recording beam due to various factors resulting from the changing of the environmental temperature has conventionally been compensated for by processing each construction element, such as a drum, with high precision.
Also, the longitudinal displacement of a recording beam due to the changing of the environmental temperature is coped with by moving the position of a lens system for imaging light emitted from a light source or by selecting an appropriate glass material for a lens.
For instance, as a method of compensating for the longitudinal displacement of a recording beam, in the case of the outer drum type, as shown in FIG. 24, light emitted from the light source 700 is converted into parallel light using the first lens group 702, this parallel light is condensed by the second lens group 704, and the condensed light is imaged on the recording medium 708 wound around the external peripheral surface of the drum 706. During this operation, in the case where there occurs longitudinal displacement of the recording beam due to the changing of the environmental temperature, as indicated by the broken line in the drawing, the longitudinal displacement is compensated for by moving the position of the second lens group 704.
Also, as a method of compensating for the longitudinal displacement, in the case of the inner drum type, as shown in FIG. 26, a laser light emitted from the light source (LD, laser diode) 710 is guided to the spinner 720 through the lenses 712, 714, 716, and 718 and the like, is reflected by the reflection surface of the spinner forming an angle of substantially 45° with respect to the optical axis, and is imaged on the recording medium 722 placed on a drum (not shown). During this operation, in the case where there occurs longitudinal displacement of a laser beam due to the changing of the environmental temperature, by selecting an optimum glass material for the lens 718 that is closest to the spinner 720 in the drawing, the longitudinal displacement is compensated for.
However, if there is made an attempt to compensate for the lateral displacement of the recording beam due to the changing of the environmental temperature by relying on the high-precision processing of a drum and the like, there occurs a problem in that the price of a product is raised.
Also, with the aforementioned method with which the longitudinal displacement of the recording beam due to the changing of the environmental temperature is compensated for by moving the position of the lens system, heavy-weighted lenses are mechanically moved, so that there is a danger that vibrations generated by the mechanical moving exert an adverse effect on recording accuracy, which causes a problem in that there is a fear of the stability of the system being lacked.
Further, with the aforementioned method with which the longitudinal displacement is compensated for by a lens glass material, there occurs a problem in that a certain restriction is put on the glass material for the lens, there occurs the lowering of the beam quality (degradation in the beam diameter or the like), and the price of a product is raised depending on the glass material for the lens.
Also, the degradation in the condensing spot shape of a light beam is caused not only by longitudinal displacement of a light beam but also by aberration in a scanning optical system or the like. Also, as a result of the changing of the shape or the characteristics of the lens glass material itself used in a scanning optical system due to the changing of the environmental temperature, the aberration in the scanning optical system also changes and the degradation state of the condensing spot shape changes. There is a problem in that in order to cope with such degradation in the condensing spot shape due to the aberration in the scanning optical system and the like and the changing of the degradation state, there is taken no effective measure other than the use of a high-priced lens glass material with less aberration.
Also, as described above, in a light beam scanning apparatus, as an image recording apparatus that records an image on a recording sheet like a recording material using a laser beam, there are used a plane scanning (flat bed) type image recording apparatus that performs recording by irradiating a laser beam onto a plane-shaped recording sheet that is conveyed for auxiliary scanning in the main scanning direction, a cylindrical external surface scanning (outer drum) type image recording apparatus that performs recording by irradiating a laser beam on a recording sheet stuck on the external peripheral surface of a rotating drum, a cylindrical internal surface scanning (inner drum) type image recording apparatus that performs recording by irradiating a laser beam on a recording sheet stuck on the cylindrical internal surface, and the like.
Among these, the inner drum type image recording apparatus has come to be frequently used because a recording sheet is stuck on the internal surface of a cylinder so that there occurs no peeling of the recording sheet during recording, the size accuracy of an image to be recorded is high, and this recording apparatus is excel in the high-speed scanning property, cost efficiency, and the like.
As described above, in the inner drum type image recording apparatus, for instance, a light scanner (spinner), whose reflection surface is set at an angle of substantially 45° with respect to the incident direction of a laser beam, is disposed on the center axis of a cylinder and this reflection surface is rotated about the center axis, thereby scanning the laser beam on the recording sheet.
Incidentally, in the inner drum type image recording apparatus constructed in this manner, there is made an attempt to improve the recording speed by performing scanning using a plurality of laser beams. In this case, if the plurality of laser beams are simply made incident on a light scanner, it is impossible to have the laser beams scan on a recording sheet in a straight manner. Consequently, it becomes impossible to perform precise image recording. If scanning lines are formed on a recording sheet using three laser beams #1 to #3 arranged in parallel at regular intervals, for instance, aside from a scanning line formed by the laser beam #2 that is made incident on the rotation center of the reflection surface, bent scanning lines are formed on the recording sheet by the laser beams #1 and #3 that are made incident off the rotation axis of the reflection surface, as shown in FIG. 27.
This bending of a scanning line is caused by the rotation of the reflection surface that has been set at an angle of substantially 45° with respect to the incident direction of the laser beam.
That is, as shown in FIGS. 28A to 28C, a light scanner 802, whose reflection surface 804 is set at an angle of substantially 45° with respect to the incident directions of the laser beams #1 to #3, is disposed on the center axis 806 of the cylinder and the reflection surface 804 is rotated about the center axis 806. As a result, in the case where the laser beams #1 to #3 are scanned on a recording sheet, when the reflection surface 804 of the light scanner 802 is set in the direction of FIG. 28A (the minor axis of the reflection surface 804 coincides with the Y axis), three laser beams #1, #2, and #3 that are made incident on the minor axis of the reflection surface 804 under a state where these laser beams are parallel to the Z axis are reflected along the X-Y plane, and reach the recording sheet S under a state where these beams advance on the same scanning line. Also, when the reflection surface 804 is rotated from the state shown in FIG. 28A by 90° and is placed in a state shown in FIG. 28B, the laser beams #1, #2, and #3 are reflected along the Y-Z plane and reach the recording sheet S under a state where these laser beams #1, #2, and #3 are separated from each other. When the reflection surface 804 is further rotated from the state shown in FIG. 28B by 90° and is placed in the direction shown in FIG. 28C, the laser beams #1, #2, and #3 are reflected along the X-Y plane again and reach the recording sheet S under a state where these beams advance on the same scanning line.
The positions of the laser beams #1, #2, and #3 on the reflection surface 804 that are made incident on the surface 804 in this manner fluctuate due to the rotation of the light scanner 802, so that aside from the scanning line formed by the laser beam #2 that is made incident on the rotation axis of the reflection surface 804, there are formed bent scanning lines on the recording sheet S, as shown in FIG. 27.
Incidentally, in order to cope with this problem in that scanning lines are bent, JP 08-130612 A discloses an inner drum type image recording apparatus that is capable of forming a scanning line that is parallel to the main scanning direction through the deflection of each laser beam that is made incident separately from the rotation axis of the reflection surface 804 using an acoustic optical device or an electrooptical device.
However, the acoustic optical device and the electrooptical device are extremely high-priced elements, so that there arises a problem in that the inner drum type image recording apparatus will cost much.
Also, in the case where the scanning position on the recording sheet S in the main scanning direction and the auxiliary scanning direction is displaced, it is required that a plurality of acoustic optical devices and electrooptical devices are used for one laser beam, which causes a problem in that the cost is further increased.